相关疑难解决方法(0)

我已经阅读了几篇文章和示例,并且尚未找到在MongoDB中执行此SQL查询的有效方法(其中有数百万个 行 文件)

第一次尝试

(例如,从这个几乎重复的问题 - Mongo相当于SQL的SELECT DISTINCT？)

db.myCollection.distinct("myIndexedNonUniqueField").length

显然我得到了这个错误,因为我的数据集非常庞大

Thu Aug 02 12:55:24 uncaught exception: distinct failed: {
        "errmsg" : "exception: distinct too big, 16mb cap",
        "code" : 10044,
        "ok" : 0
}

第二次尝试

我决定尝试做一组

db.myCollection.group({key: {myIndexedNonUniqueField: 1},
                initial: {count: 0}, 
                 reduce: function (obj, prev) { prev.count++;} } );

但我收到此错误消息:

exception: group() can't handle more than 20000 unique keys

第三次尝试

我还没有尝试,但有几个涉及的建议 mapReduce

例如

• 这一个如何在mongodb中做出明显的分组？(不接受,答案作者/ OP没有测试)
• 这个MongoDB group by Functionalities(看起来类似于Second Attempt)
• 这个http://blog.emmettshear.com/post/2010/02/12/Counting-Uni​​ques-With-MongoDB
• 这一个https://groups.google.com/forum/?fromgroups#!topic/mongodb-user/trDn3jJjqtE …

如何获取包含集合中所有文档的某个字段的所有值的数组？

db.collection:

{ "_id" : ObjectId("51a7dc7b2cacf40b79990be6"), "x" : 1 }
{ "_id" : ObjectId("51a7dc7b2cacf40b79990be7"), "x" : 2 }
{ "_id" : ObjectId("51a7dc7b2cacf40b79990be8"), "x" : 3 }
{ "_id" : ObjectId("51a7dc7b2cacf40b79990be9"), "x" : 4 }
{ "_id" : ObjectId("51a7dc7b2cacf40b79990bea"), "x" : 5 }

"db.collection.ListAllValuesForfield(x)"结果:[1,2,3,4,5]

此外,如果这个字段是一个数组怎么办？

{ "_id" : ObjectId("51a7dc7b2cacf40b79990be6"), "y" : [1,2] }
{ "_id" : ObjectId("51a7dc7b2cacf40b79990be7"), "y" : [3,4] }
{ "_id" : ObjectId("51a7dc7b2cacf40b79990be8"), "y" : [5,6] }
{ "_id" : ObjectId("51a7dc7b2cacf40b79990be9"), "y" : [1,2] }
{ "_id" : ObjectId("51a7dc7b2cacf40b79990bea"), …

我的actors集合包含一个文档数组字段，称为acted_in. 我不想返回像这样的大小acted_in.idmovies：{$size: $acted_in.idmovies}，我想返回内部不同值的数量$acted_in.idmovies。我怎样才能做到这一点 ？

c1 = actors.aggregate([{"$match": {'$and': [{'fname': f_name},
                                            {'lname': l_name}]}},
                       {"$project": {'first_name': '$fname',
                                     'last_name': '$lname',
                                     'gender': '$gender',
                                     'distinct_movies_played_in': {'$size': '$acted_in.idmovies'}}}])

{

    {
    "date": "2017-09-04",
    "description": "DD from my employer1",
    "amount": 1000.33
    },
    {
    "date": "2017-09-06",
    "description": "DD from my employer1",
    "amount": 1000.34
    },
    {
    "date": "2017-09-06",
    "description": "DD from my employer1",
    "amount": 1000.35
    },
    {
    "date": "2017-09-07",
    "description": "DD from employer1",
    "amount": 5000.00
    },
    {
    "date": "2017-09-08",
    "description": "DD from my employer1",
    "amount": 2000.33
    },
    {
    "date": "2017-09-09",
    "description": "DD from my employer1",
    "amount": 2000.33
    },
    {
    "date": "2017-09-10",
    "description": "DD from my employer1",
    "amount": 2000.33
    }

}

我上面有这组对象，我正在尝试计算唯一日期的数量。有 7 …